The invention is based on a priority application EP 03 360 051.1 which is hereby incorporated by reference.
The invention relates to a receiver device for optical data signals, in particular optical data signals in the Gb/s range, comprising an opto-electrical conversion unit, a frequency multiplicator unit for frequency-multiplying the converted electrical data signal, and a clock recovery unit.
A receiver device of such type is disclosed in G. P. Agrawal, Fiber-Optic Communication Systems, 2nd edition, Wiley, N.Y., 1997, page 159-160.
Devices for reading out optical data signals consist of an opto-electric conversion unit, a decision circuit and a clock recovery circuit. The clock recovery circuit's purpose is to isolate the spectral component at the bit rate frequency B. In return to zero (RZ) signals, the data signal contains a clock line at the bit rate frequency, and the clock line can be isolated by appropriate band pass filtering. The clock line is used for timing the decision circuit.
In non return to zero signals (NRZ), the data signal does not contain a clock line at the bit rate frequency. In this case, a frequency doubling (sometimes referred to as squaring) is performed with the received optical signal after its conversion into an electric signal. The spectral components originally symmetrically around half the bit rate frequency B/2 are used as the clock line then. A similar procedure can be performed with RZ signals if the clock line in the original signal is too weak for some reason.
When transmitting high bit rate optical signals in optical fibers over long distances, the optical signal is subject to dispersion, in particular polarization mode dispersion (PMD) and chromatic dispersion (CD). These dispersion effects reduce the optical power within the transmitted optic signal, in particular in the frequency range of half the bit rate B/2. As a result, after frequency doubling, the clock line at the bit rate frequency B is very weak, often too weak for a reliable data recovery. For this reason, expensive dispersion compensation equipment has to be applied in order to improve the clock line generation.